Lignin is a complex chemical compound most commonly derived from wood and one of the most abundant organic polymers on Earth. Lignin is typically deposited in nearly all vascular plants and provides rigidity and strength to their cell walls. As a biopolymer, lignin is unusual because of its heterogeneity and lack of a defined primary structure.
Lignin refers to a biomass material which is a cross-linked racemic macromolecule with molecular masses in excess of 10,000 u. It is relatively hydrophobic and aromatic in nature. The degree of polymerisation in nature is difficult to measure, since it is fragmented during extraction and the molecule consists of various types of substructures which appear to repeat in a haphazard manner.
The lignin polymeric structure is composed primarily of three phenylpropanoid building units: p-hydroxyphenylpropane, guaiacylpropane, and syringylpropane interconnected by etheric and carbon-to-carbon linkages. Generally, in unprocessed lignins, two thirds or more of these linkages are ether bonds, while the remaining linkages are carbon-carbon bonds. Different types of lignin differ significantly in the ratio between these monomers.
Numerous efforts on lignin conversion to high-value fuels and fuel additives have included a number of single stage processing methods including hydrocracking, cracking, hydrogenation, hydrotreating, liquefaction in hydrogen-donor solvents, and the like. However, to date, these approaches have achieved limited success for a variety of reasons. During the past few years several studies have been initiated to develop two-stage processes for making liquid fuels and fuel additives from lignin.
Further, there is ample literature available on depolymerization of lignin. The vast majority of lignin research in terms of transportation fuel production is done that utilize gaseous hydrogen and different catalysts to remove the covalently bound oxygen as water.
References may be made to U.S. Pat. No. 6,214,976, wherein inventor discloses a process for depolymerizing and/or chemically modifying lignin or lignin-like compounds using a solution of copper and peroxide at a temperature of less than 100° C.
References may be made to U.S. Pat. No. 5,959,167, wherein inventor describes a process for converting lignin into reformulated hydrocarbon gasoline, comprising subjecting the lignin material to a base-catalyzed depolymerization to produce a depolymerized lignin product consisting of alkylated phenols, alkylated alkoxyphenols, hydrocarbons, and mixtures thereof; Further, the depolymerized lignin product is subjected to a sequential two-step hydroprocessing reaction (HPR) to produce a reformulated hydrocarbon gasoline product.
References may be made to U.S. Pat. No. 6,172,272, wherein inventor discloses a process for converting lignin into reformulated partially oxygenated gasoline comprising subjecting the lignin material to a base-catalyzed depolymerization reaction in the presence of a supercritical alcohol at 240° C.-270° C.; followed by selective hydrocracking reaction at 340° C.-375° C., which utilizes superacid catalyst (platinum-modified catalyst) to produce a high oxygen-content depolymerized lignin product mainly composed of alkylated phenols, alkylated alkoxyphenols, and alkylbenzenes. In the second stage of the process, the depolymerized lignin product is subjected to an exhaustive etherification reaction with MeOH in presence of a catalyst; optionally followed by a partial ring hydrogenation reaction to produce a reformulated, partially oxygenated/etherified gasoline product, which includes a mixture of substituted phenyl/methyl ethers, cycloalkyl methyl ethers, C7-C10 alkylbenzenes, C6-C10 branched and multibranched paraffins, and alkylated and polyalkylated cycloalkanes.
References may be made to U.S. Pat. No. 6,100,385, wherein inventor discloses a method of producing lignin phenol from lignin, which comprises placing lignin in contact with a hydrogen donor organic solvent such as tetralin, lignin phenol or biomass oil; converting the lignin into low molecular weight-lignin phenol by reducing the lignin in the presence of a reducing agent (such as CO, H2, H2S and mixture thereof) and a catalyst at a temperature preferably about 300° C. to about 550° C., wherein said catalyst contains molybdenum oxide and a second component selected from the group consisting of nickel oxide or cobalt oxide supported on alumina (neutral, acidic or basic).
References may be made to US2008050792, wherein inventor discloses a process for conversion of a lignin material to a bio-fuel, comprising subjecting the lignin material to a base catalyzed depolymerization reaction at a temperature of 230° C.-350° C. to produce a partially depolymerized lignin, which is a mixture of monomeric and oligomeric alkylated phenols, biphenols, alkoxyphenols and some hydrocarbons; subjecting the partially depolymerized lignin to a stabilization/partial hydrodeoxygenation reaction in the presence of a solid metal catalyst containing one or more metals selected from the group consisting of Group VI and Group VIII metals of the Periodic Table and vanadium and combinations thereof on a solid support at a temperature of about 200° C. to 300° C. to form a partially hydrodeoxygenated product; followed by hydroprocessing step to yield bio fuel.
References may be made to WO2009021733, wherein inventor discloses a method of converting a lignin material into a liquid product, wherein lignin material is subjected to a treatment in a reaction medium of at least one C1-C2 carboxylic acid, and/or salts and/or esters thereof; optionally at least one alcohol and/or water; optionally an alkylating agent, and optionally an inorganic salt under elevated pressure to yield a liquid product. The said method is carried out in the absence of added gaseous H2 and/or in the absence of any added catalyst at reaction temperature from 320° C.-420° C.
References may be made to US2010137663, wherein inventor relates to a process for producing chemicals from lignin comprising: combining lignin with a hydrogenation catalyst under hydrogen atmosphere along with base selected from the group consisting of alkali metal hydroxides, alkaline earth metal hydroxides, alkaline earth metal oxides, alkali metal carbonates, and mixtures thereof; reacting the mixture in lignin hydrolysis process conditions at a temperature between 250° C. and 450° C. to form an intermediate product stream having a reduced oxygen content; and contacting the intermediate product stream with a deoxygenation and a dehydrogenation catalyst at dehydrogenation reaction conditions including a temperature of between 400° C. and 900° C. thereby generating a product stream comprising aromatic compounds.
These methods are not environmentally benign and are energy consuming. Hence, it is required to develop a process which will not use soluble base, will operate at milder reaction conditions, and will be environmentally benign.
References may be made to US2010137665, wherein inventor discloses a process for the production of hydrocarbons and hydrogen from lignin wherein the first step comprises combining lignin with water and an acidic or basic catalyst in the presence or absence of hydrogen atmosphere to form a mixture of hydrocarbons and light oxygenates with some undecomposed lignin. For acidic depolymerization, it is preferred to have a hydrogen atmosphere. The second step involves decomposing light oxygenates to hydrogen and carbon dioxide in presence of a decomposition catalyst to form a second intermediate stream comprising hydrogen; followed by a third step of combining second intermediate stream with a hydrogenation catalyst to form decomposed lignin, hydrogenated or partially hydrogenated compounds; further contacting the third stream with a dehydrogenation catalyst to form a stream comprising aromatic compounds. It is a process to produce hydrocarbons from lignin, without the addition of an external source of hydrogen, the process itself generates hydrogen that is used in the process.
However, there is still a need in the art for an improved and environment friendly process for depolymerization of lignin which will ameliorate the drawbacks of existing prior processes.